#include<stdio.h>
#include<math.h>
int main()
{   float a, b, c, p,S;
	scanf("%f%f%f%f",a,b,c);
	p = (a+b+c)/2;
	S = sqart(p*(p-a)*(p-b)*(p-c));
		printf("%f",S);
	return 0;




}
